Asked by Apr 26, 2014 at 09:17 PM about the 1967 Ford Mustang Convertible

Question type: Maintenance & Repair

What rear differential gear ratio would you recommend to give the best low end power from
a stop as well as being ok while cruising at highway speeds (3:55 or 3:73 or ??).

2 Answers

Paolo Tomaso

It depends on your tranny, do you have an overdrive, and your definition of "ok at highway speeds".


If your car has a Factory 3 or 4 speed, OR a C4 Automatic... Then 3:55's and 3:73's are a poor choice. Those Ratios were never available in 1967, mainly because 67's didn't use Overdrive Transmissions like the later year Mustangs did that used those Ratios. The Better options to consider are a 3:25 & 3:50 Ratios for a performance application in a 67 with a Factory 3 or 4 speed (or C4 Automatic). The thing you'll use to guide you is the Dia. of your rear tires.. or, how TALL they are. If your rear tires are 26" tall (or taller), than you'll use the 3:50 Ratio. If the rear tires are LESS than 26" tall, use the 3:25 Ratio. If you had an original 67 Mustang GT, The ratio would have been (standard) 3:25... your car would have came with a 26" tall Bias ply Firestone tire on it. I'd say it's likely that you are using Modern Radials on your car and your tires are LESS than 26" tall...therefore, JUST by using a shorter tire, you are UPing the final drive ratio to some extent. This causes the 3:25's to give engine rpm's similar to the 3:50's. Remember, 3:73's were used in a car with overdrive... in overdrive the 3:73's act like 3:40's (?? in that range). My opinion is that if you use 3:55's or 3:73's you're NOT going to like the Rpm you're operating at doing 70 on the highway, NOR are you going to be happy with fuel mileage you'll be getting. My opinion is to go with 3:25's with a stock (Non-overdrive trans), and modern radials under 26" tall. You'll definitely be able to get performance increases over the 2:79's in your car now and not sacrifice fuel economy to the point it hurts.

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